A conventional automatic vehicle-body assembling system of the nature above specified is composed solely of a number of multi-spot welding machines arranged along an assembly line. Each of these welding machines includes positioning means adapted to hold together a plurality of component assemblies to constitute a vehicle floor structure and jig units including spot welding heads for spot welding together the component assemblies thus positioned with respect to each other. In this instance, the jig units are arranged so that the welding heads are operative to have the component assemblies of the vehicle floor structure spot welded simultaneously at a number of locations of the component assemblies. The jig units of the welding machine are designed depending upon the types, models and/or makes of the automotive vehicles to be manufactured and, for this reason, must be utilized for the assembling of vehicle floor structures of a single specified type, model or make. When the automotive vehicles to be supplied from an automotive manufacturer are to be re-modelled or modified in type or make, therefore, the existing assembly line which has been put to use sometimes becomes useless so that it may be required for the automotive manufacturer to have the existing assembly line supplanted by an up-to-date version of the assembly line or shut down for an extended period of time until the jig units which have been used in the obsolete assembly line are completely exchanged with new ones. Problems have therefore been encountered in that a vast amount of investment is required on the part of the manufacturer or in that the labor and production equipment are wasted for a long period of time. The present invention contemplates elimination of these problems by provision of an improved automatic vehicle-body assembling system.